AB763 two channel

[Rockranger]

Hey guys just finished building the Hoffman AB763 two channel amp. I had built the 65 deluxe reverb a couple of months ago because a friend gave me all the pcb boards they gutted out of one for free, got it working but it was noisy and I wasn’t happy with the sound so I decided to rip that stuff out and do the Hoffman AB763 design. Just wondering if someone has all the appropriate tube voltage #’s, b+ voltages and proper bias numbers for this design I wasn’t able to find it on the site. Everything fired up when I turned it on but I have no sound and low b+ and plate voltage (255 VDC) so I have some troubleshooting to do!

Thanks

[sluckey]

I have a single channel with reverb, but no tremolo. Voltages are on the schematic and should be helpful to troubleshoot your amp. Your voltages should be similar. I suggest you look for wiring errors. Did you use a Hoffman board and follow his layout?

     http://sluckeyamps.com/tdr/tdr.pdf

[Rockranger]

Yes I used the Hoffman board and followed his layout, thank you for the schematic with the voltages. Should be able to go through and figure it out things are much more messy than I’m used to where I did the reissue first then ripped everything out. Thanks!

[Rockranger]

I am using Hammond transformers from the previous build and I have a red/yellow 50v tap that isn’t being used just wondering what the best thing to do with this is? (Could this be one of my problems). Seems like all my voltages are half of what they should be for example V1 pin  1 103, pin 3 0.77, pin 6 101 and b+ is close to half.

[shooter]

Could this be one of my problems

typically no, unless you messed up somehow whatever you are using for bias.
I just shrink tube the "unused wire", fold it back on itself and shrink tube again, then zip-tie outta the way

[bnwitt]

You might want to start by filling out a blank chart first to share and take some photos of the build

[bnwitt]

Here is my layout which is the same as Hoffman's with a little more shown

[sluckey]

I am using Hammond transformers from the previous build and I have a red/yellow 50v tap that isn’t being used just wondering what the best thing to do with this is? (Could this be one of my problems). Seems like all my voltages are half of what they should be for example V1 pin  1 103, pin 3 0.77, pin 6 101 and b+ is close to half.

STOP! Are you sure that red/yellow is a bias tap. Red/yellow is usually the center tap. What is the Hammond part number for that power transformer?

[Rockranger]

It is a Hammond 290BX. Says on it that red/yellow-blue: 50v

[sluckey]

The Red/Yellow wire is zero volts. It's the center tap. IT MUST CONNECT TO CHASSIS GROUND!

The Blue is the 50VAC. It's the bias wire. IT MUST CONNECT TO THE "BIAS TAP" POINT ON THE BOARD. (purple wire)

Here's the data sheet for that PT...

     https://www.hammfg.com/files/parts/pdf/290BX.pdf

[Rockranger]

Ok thanks for everything guys I’m gonna spend a couple of days making sure everything is straightened out!

[shooter]

STOP!

  good catch, you are the master 

[Rockranger]

I had some confusion with the mess I am making out of this my center tap was grounded I didn’t change anything that didn’t have to be changed from the reissue build(which was working) guess it was just the way the wording on the transformer looked like there was another wire. Anyway I took a close look at things and found a ground wire that wasnt soldered properly and was separated from ground buss, cleaned up a few things and fired up, got a loud noise as soon as I flipped the stby switch so I reversed the OT To pin3 of the output tubes, didn’t help so I flipped them back. Power tubes got real hot, looked like they were starting to glow a bit so I turned everything off right away. Desoldering everything and took the board out to go through everything here. Just noticed I have an unused .047 cap here everything else on the parts list has been used, it says there are 4 .047 caps on the list and I only see use for 3 on the left side of the board, anyone know where the 4th goes?

[sluckey]

Just noticed I have an unused .047 cap here everything else on the parts list has been used, it says there are 4 .047 caps on the list and I only see use for 3 on the left side of the board, anyone know where the 4th goes?

Hoffman's layout, schematic and BOM all show only one .047µF cap.

What are you looking at that says there are four .047 caps?

[Rockranger]

The parts list for the AB763 has 4 and the Hoffman AB763 PDF file shows 3 for sure on the board.
http://hoffmanamps.com/MyStore/PartsListPublic.php?action=public&p=N&f=Amp_AB763TwoChannelParts_DR
http://el34world.com/schematics.htm#Hoffman_AB763_Two_Channel_

[sluckey]

Sorry, you're right. I was looking at a Princeton Reverb for some stupid reason! 

Hoffman's pdf only shows three .047s. His BOM also correctly shows three .047s.

     http://el34world.com/Hoffman/files/Hoffman_AB763_2_BOM.pdf

However, that ordering list does have a default QTY of 4. I've never looked at that ordering parts list. It does say at the top that you can change the quantity for any item. If you mention that to Doug he may change the order list to default to QTY 3.

[Rockranger]

Thanks for looking into it just wanted to make sure I wasn’t missing something obvious.

[Rockranger]

Hey guys, I gutted this thing and put it all back together and I finally have a working amp, but I have problems in the output section my plate voltages are high and I have off numbers on pin 5 and pin 8.
Board A: 463vdc
Board B: 463
Board C: 365
Board D: 303

V7 & V8
Pin 3: 463
Pin 4: 460
Pin 5: -55
Pin 8: 0.9ma

I have bias test points set up on this amp but that is obviously way low with my pin 8 value being what it is any suggestions would greatly be appreciated, 24 ma is the norm on pin 8 why would I barely be registering a number on the cathode, my bias pot swings the value just slightly?

[sluckey]

Your bias voltage is too high and is causing the other voltages to be off. Increase the value of the 470Ω bias range resistor. I'd try 1K to start. See if your voltages get more right.

[Rockranger]

Thanks sluckey hopefully that will do it I can’t be too far off, I don’t have any 3w resistors so I will order in some and let you know how it goes thanks!

[sluckey]

Get several values. Two watters work fine too. In fact, the original Fenders used a 470Ω/1W in that spot.

While you wait for your order to come in, pull your output tubes and connect your voltmeter to pin 5 of one of the output tubes. Now turn the bias pot from end to end. What are the voltage readings at each end of rotation? And what power tubes are you using?

[Rockranger]

Yeah I ordered common values from 820 to 2k. Pulled my output tubes and bias pot swings from -25 to -71 on pin 5, I am using JJ 6V6S!

[sluckey]

Yeah I ordered common values from 820 to 2k. Pulled my output tubes and bias pot swings from -25 to -71 on pin 5, I am using JJ 6V6S!

I don't think you will need those resistors. You have enough bias pot swing to properly bias any tube that will fit that socket. You need to look for another problem.

[Rockranger]

Ok I’ll keep looking, I really gave this a good look over the last couple of days hopefully I can come up with something!

[Rockranger]

This project had become so messy I figured I should just take everything apart get some fresh wire and clean it up. The amp if working perfect as far as voltages go and everything seems great and as it should be, all the problems I was having are gone. The reason I redid this amp from the reissue version I had with all the pcb board was because of the noise. My house is quite noisy for amps but this one was exceptionally noisy and after the AB763 rebuild it still is very noisy, the reverb is almost unusable it adds that much noise to the signal! Knowing that I have a noisy house I have kept it in mind during the build (short low voltage leads, neat heater wiring, copper tape on my amp cabinet where the chassis is placed......) Anyone with any ideas how to quiet this down any would be greatly appreciated, my shielded wires from my input and volume are running under the board should this be changed? There are long leads going from the board to vibrato channel pots would adding any shielded wire anywhere else help? Normal channel is sounding the best, vibrato channel gets worse and adding reverb makes it way worse!
With no input and no volume on there is noise, considerably better without V4, no noise without V6, I’m not worried about the power section, just want to clean up the reverb vibrato section!

[Rockranger]

After cleaning things up some more and shotening up a few leads and making my own rca cord (way shorter than the one I purchased) I cleaned up the sound pretty good, the normal channel is perfect! Played it steady over the past day or so then I blew a fuse, replaced it and blew another one noticing that the rectifier was sparking inside! Just wondering what would cause this after having it working perfectly with all the proper voltages? And as a side note just wondering what fuses to use in this amp I had 1A slow blow in it that I got when I ordered stuff from the parts list for the AB763 project but the layout diagram says 3A slow blow?

[sluckey]

Refer to the schematic... The fuse is one of those items that varies with the model you build. Hoffman's BOM specifies 1A SLO, same as the original AB763 DR.

I've seen several GZ34s sparkle inside. I always replace them when I see that.

[Rockranger]

Right on I thought it may just be the rectifier itself you think a new one will solve my blown fuse problems?

[Rockranger]

Found an old rectifier tube and started running some tests. I have 380v coming from the rectifier to the standby switch as soon as I flick the standby switch I voltage beginning to climb on board A once it hits about 190v it starts falling backwards and the rectifier begins to get really hot so I don’t have much time to test, no voltage reaches board B. Just wondering what could cause this, choke seems to be getting hot as well, keep in mind this amp worked perfectly for a couple of days!

[sluckey]

Pull the output tubes and don't put them back in until we resolve this problem. Disconnect both leads of the choke. Can you now get proper voltage at Board A?

[Rockranger]

Thanks for the response sluckey just heading to work night shift I’ll be back at it tommorow!

[Rockranger]

Took the output tubes out disconnected both leads of the choke, board A goes straight to 500v and stays there, put the rectifier that was sparking in for this and it is fine with the choke disconnected not heating up at all.

[sluckey]

OK, now reconnect the choke and disconnect the resistor between "B" and "C". Do you now get good voltage at "A" and "B"? Power tubes still out.

[Rockranger]

Choke reconnected, power tubes out and resistor removed between B and C, I get the same problem voltage jumping everywhere on A none on B and rectifier overheating.

[sluckey]

Good. We're only a couple minutes away from solving this. Leave the resistor disconnected for now. Disconnect the positive end of the filter cap from "B". Do you now get good voltage at A and B?

[Rockranger]

Disconnected the positive side of B filter cap and have the same issue voltage everywhere on A none on B.

[sluckey]

Leave the cap disconnected. Now disconnect both of those 470Ω/3W resistors that connect to point B. Got good voltage now?

[Rockranger]

Cap still disconnected 470 ohm resistors disconnected same issue.

[sluckey]

Leave all disconnected. Not much left now. Use your ohm meter to check resistance between board B and chassis. Meter should read very high or open. What have you?

There are two underboard jumpers connected to B. If you have a low resistance reading, I suspect one of those jumpers is somehow shorted to chassis or is not connected to the proper turret. Take a close look.

[Rockranger]

Checked board B and it was going to ground, removed both wires that were running underneath the board connected them back in above and everything was fine. Put it all back together and everything is back to normal, I must have created a bad connection where board b connects to the red wire from the reverb driver when I was messing around trying to clean up vibrato channel noise! Really appreciate the help sluckey you saved me hours!!!!

[dude]

So your problem was loose, bad connection, or the like of high voltage wires under BD?

[Rockranger]

Bad connection under the board where the reverb driver red wire meets the board!

[dude]

Next build, a good way to check your bd after done is to take a non-permanent magic marker and follow the layout from one end to the other, putting a mark on each connection. Go slowly, maybe do it twice, once starting at one side of Bd, then the other. Make sure you have marks on all connections. For under the Bd wiring use you meter, ohms and continuity, and check to ground for continuity (if it beeps, which it probably would have in you case) redo that wire.


Sluckey is good, has a lot patience. I'm glad you found your problem.


al

[Rockranger]

Yeah I had taken the precautions and took this amp apart three times before I got it working properly  . I had noise in the vibrato channel and was messing around with the reverb section ( didn't realize I knocked that wire out from under the board when I removed the reverb driver wire )With all the solder this project has seen I just wanted to solve the issue as quickly and cleanly as possible, really appreciate sluckeys expertise!!!!Thanks

[PRR]

> took this amp apart three times before I got it working properly

I just took a TOILET VALVE in and out THREE times before I got it right.

And your amp isn't (I hope) full of COLD water. (Well, the toilet was near-dry, cuz it all ran on my hands and toes.)

> didn't realize I knocked that wire out

Then there is the wire I knocked out of (actually guillotined) a brand-new $408 yard-lamp today , but I'm not ready to tell that tale.

[sluckey]

( didn't realize I knocked that wire out from under the board when I removed the reverb driver wire )

I don't like using underboard jumpers but sometimes it's unavoidable with a complicated board. When I do use them I want to know they will stay in place securely. So, I always push the end of the jumper completely through the turret and 'hook' the end back over the outside of the turret. This little detail allows you to see the jumper even after soldering, but more important, it prevents the jumper from falling out of the turret whenever you do any later soldering.

[Rockranger]

Definitely should have kept the underboard connections above there was enough room, I was using a lot of left over stuff I had for this project and ended up using eyelets for the board, did a couple of previous builds that were all turrets this one is not as neat and the connections aren’t as stable. Really should have had everything above the board!